According to Fortune, AT&T's going to further subsidise the iPhone down as much as US$200, making the final retail price a scant US$199 with two-year-contract. The rumour is that the US$200 bonus will only be available in AT&T stores, not in Apple stores, so people buying and exporting or using it with T-Mobile will be paying the higher price. Current first-gen iPhone users probably won't get this US$200 subsidy when upgrading either, unless perhaps they renew for another 2 years.
If you're buying straight from Apple, it seems like their prices will still be US$399 and US$499 for the 8 and 16GB models. Fortune's source also says that the phone will have GPS and be 2.5mm thinner, not thicker as previously indicated. And you'll know when the new phone is coming—Apple will cut off shipments of the old phone in order to clear up inventory and give a subtle hint that you should wait before buying. [Fortune]